Referring to FIG. 1, which shows a conventional analog audio processing circuit, wherein a signal S is inputted to the left side of an input buffer 1, and then into an output buffer 4 after passing through an audio processing circuit 2 and a switching circuit 3. The switching circuit 3 can optionally input the output B of the audio processing circuit 2 or the output A of the input buffer 1 into the input D of the output buffer 4. A voltage reference circuit 5 is used to provide a reference voltage C for the input buffer 1, the audio processing circuit 2 and the output buffer 4.
Generally, the input buffer 1 is an operational amplifier, and so is the output buffer 4. The audio processing circuit 2 is used for processing the volume, alt, bourdon, surround sound effect, stereo, etc, generally includes six operational amplifiers.
Each of the operational amplifiers generates offset voltage. Suppose that the offset voltage ΔV is 10 mV, the reference voltage C of the voltage reference circuit 5 is 4.5 V, then:
the output A of the input buffer 1 =C+ΔV=4.510V.
the output B of the audio processing circuit 2=C+6×ΔV+X (X is generated by the surround sound effect and the stereo sound, and is about 50 mV), so B=4.610V
This means that when the switching circuit 3 switches between the output B of the audio processing circuit 2 and the output A of the input buffer 1, it will generate a 100 mV voltage difference, and this is the reason why a sound like “bo” will always be caused when the user turns on/off the switching circuit 3, and this is so-called pulse noise, music player's headache.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.